1. Technical Field
The present invention relates to a pressure detecting device.
2. Related Art
JP-A-2007-327922 discloses a pressure sensor including a diaphragm, a container, and a double-ended tuning fork type resonator (pressure-sensitive element) mounted on a support portion of the diaphragm.
In such a pressure sensor, a flexible diaphragm is deformed with a pressure applied to a pressure-receiving face, tensile stress (extensional stress) or compressive stress is applied to two vibration beams of a double-ended tuning fork type resonator, and the magnitude of the pressure applied to the pressure sensor is detected by measuring the oscillation frequency (resonance frequency) of the double-ended tuning fork type resonator which varies depending on the tensile stress or the compressive stress.
In the pressure sensor, when a double-ended tuning fork type resonator using quartz crystal as a base material is used as a pressure-sensitive element, a quartz crystal wafer called Z plate and cut perpendicular to the Z axis is typically used as a quartz crystal substrate.
Paying attention to the frequency-temperature characteristic of the double-ended tuning fork type resonator, the frequency-temperature characteristic of the double-ended tuning fork type resonator is equivalent to the frequency-temperature characteristic of a tuning fork type resonator. Regarding the relation between the frequency-temperature characteristic and the cut angle of the tuning fork type resonator, as described in JP-A-2005-197946, it has been known that the cut angle of the quartz crystal substrate is an angle θ (where θ is in the ranges of 0 to ±15°, 15° to 25°, 30° to 60°, and the like) by which the XY plane (Z plate) is rotated about the X axis, the resultant tuning fork type resonator vibrates in a flexural vibration mode and the graph representing the frequency-temperature characteristic is a quadratic curve.
Since the frequency-temperature characteristic is expressed by a quadratic curve having a peak in the vicinity of the ordinary temperature, the variation in frequency with the temperature in the vicinity of the ordinary temperature is small.
In order to compensate for the frequency-temperature characteristic of the double-ended tuning fork type resonator in detecting a pressure, a temperature sensor is provided and the detected pressure is corrected on the basis of information from the temperature sensor (for example, see High-precision Pressure Sensor using double-ended tuning fork type crystal resonator (Papers of Epson Toyocom, 38th EM Symposium, May 14, 2009)).
In the pressure sensor, a reference frequency source outputting a reference clock signal is necessary for measuring the oscillation frequency of the double-ended tuning fork type resonator. Since the precision in measuring a pressure depends on the precision of the reference frequency source, a high-precision oscillator such as a temperature-compensated crystal oscillator (hereinafter, referred to as “TCXO”) including an AT-cut quartz crystal resonator and a temperature-compensating IC is typically used as the reference frequency source.
The reciprocal counting method described in Japanese Patent Nos. 3931124 and 2742642 is employed as a counting method for measuring the frequency. That is, a gate period corresponding to plural periods of a signal output from an oscillation circuit causing the double-ended tuning fork type resonator as a pressure-sensitive element to oscillate is set, the reference clock signal output from the reference clock oscillator in the gate period is counted, the frequency of the signal output from the oscillation circuit is calculated on the basis of the counted value, and the frequency is converted into a pressure value, whereby the pressure is measured. Accordingly, it is possible to reduce the time required to measure a pressure.
However, in such a pressure sensor, the measurement precision is greatly affected by the frequency of the reference clock signal of the reference clock oscillator. When an error exists in the reference clock signal, the counted value of the reference clock signal in the gate period departs from an appropriate value, thereby lowering the precision in measuring a pressure. When the pressure is constant but the temperature varies, the frequency of the reference clock signal of the reference clock oscillator varies accordingly, thereby further lowering the precision in measuring a pressure.
Accordingly, a high-precision oscillator such as a TCXO including a temperature-compensating IC is used as the reference clock oscillator of the pressure sensor.
When the TCXO is used as the reference clock oscillator, for example, when the ordinary-temperature deviation in the frequency of the reference clock signal of the TCXO is ±2 ppm (which is a typical value of communication TCXO), the measurement error of the pressure sensor is ±4 Pa (where the sensitivity of the pressure sensor is 500 ppm/kPa).
For example, when the deviation in frequency of the reference clock signal of the TCXO due to the variation in temperature is ±1 ppm, the measurement error of the pressure sensor increases by ±2 Pa and the resultant measurement error is ±6 Pa.
When the size of the pressure sensor is reduced, the frequency sensitivity (sensitivity) of the pressure sensor to the pressure decreases. Accordingly, the influence of the error of the reference clock signal on the pressure measuring precision increases.
For example, when the sensitivity of the pressure sensor is changed from 500 ppm/kPa to 100 ppm/kPa, the measurement error based on the ordinary-temperature deviation and the measurement error based on the deviation due to the variation in temperature are raised to five times.
Since the TCXO includes the temperature-compensating IC, there is a problem in that the power consumption in the temperature-compensating IC is high and the total power consumption of the pressure sensor is high accordingly.